Sometimes you need a totally opaque “constant” – a value that isn’t intended to be projected or modified, and whose only purpose is to be completely different from every other value1. For example, Functional uses Functional._ as a placeholder; a comment on John Resig’s blog suggests defining something like Partial.PLACEHOLDER for something similar.

In JavaScript, these are easy to make. Here’s one: {}. And here’s another: {}. Note that these two values are different: the following code2 will print true, then true, then false:

var L1 = {};
var L2 = {}; == L1);
>>> true == L2);
>>> true == L2);
>>> false

The problem with these values is that they look the same when you print them. L1 and L2 both print as Object (in Firefox).

I’m going to print a value now: ? L1 : L2);
>>> Object

Quick, which one did I print? Sure, you can figure it out in this case (assuming my implementation of isPrime isn’t buggy – probably not a safe bet, especially if you’re having to debug this in the first place), but in general this wreaks havoc with debugging.

Here’s an idiom for making opaque values that can be debugged. This has the further benefit that if the value is bound to a variable, you can use this to create a value that evaluates to itself when you type it back into the console (or into your source code). This works in Firebug and Rhino and OpenLaszlo, at least.

var L1 = {toString:function{return "L1"}};
var L2 = {toString:function{return "L2"}};
>>> L1
>>> L2

If you do use opaque constants often, you can use this makeLiteral utility routine to make them:

function makeLiteral(name) {return {toString:function(){return name}}}
var L1 = makeLiteral("L1");
var L2 = makeLiteral("L2");
>>> L1
>>> L2

Some real-world uses might be:

Functional._ = makeLiteral("Functional._");
Partial.PLACEHOLDER = makeLiteral("Partial.PLACEHOLDER");

In fact, you could go further and define a defining-form. I’m just including this for completeness; the version here doesn’t work unless target itself has a toString() method, and would need more work to be made robust.

function defineLiteral(target, name) {
  target[name] = return {toString:function(){return target+"."+name}}
defineLiteral(Functional, '_');

  1. Basically an enumerated type or a member of an algebraic data type, except that in meta-level programming these values are often compared to any other value, not just values of a specific type. 

  2. This would work the same way with === instead of ==, but here it’s not necessary.